This invention relates to a muting circuit for an FM receiver for relieving adjacent-channel interference produced by local noises during channel selection. More particularly, it relates to a muting range setting circuit in the muting circuit for setting a muting range in which a muting operation is effectuated.
FIG. 1 shows a block diagram illustrating one example of a normally used muting circuit with accompanying circuits. In that Figure, an FM detecting circuit 1 is shown having a function to be described later. Although an antenna, a front end section, an intermediate frequency amplifier (hereinafter referred to merely as IF amplifier when applicable) and the like are not represented in FIG. 1, in an FM receiver, an FM signal received by the antenna is applied to the front end section where it is tuned to a particular broadcast frequency which is in turn converted into an intermediate frequency signal (hereinafter referred to merely as an IF signal when applicable) of 10.7 MHz. These are all standard components to such tuner sections.
This IF signal is applied to the FM detecting circuit 1 through the IF amplifier. In the situation when the input signal of the FM detecting circuit 1, which is delivered from the IF amplifier, deviates from the intermediate frequency of 10.7 MHz, the D.C. output signal of the FM detecting circuit 1 varies in response to the frequency deviation therefrom. The D.C. output signal is a so-called "S curve voltage signal". This output is fed through an AFC circuit 2 to a muting range setting circuit 4 where a muting range in which a muting operation is accomplished, is set. The output of the muting range setting circuit 4 is fed to a gate driving circuit 5. This constitutes a muting driving circuit together with the AFC circuit 2, the muting range setting circuit 4 and a gate circuit 7, to control the operation of a low-frequency amplifier 3. In addition, a D.C. reference voltage source 6 constitutes a part of the muting circuit together with the above described muting driving circuit.
In the muting circuit of the conventional FM receiver, a muting range setting circuit comprises NPN transistors and PNP transistors. In many situations, both the NPN transistors and the PNP transistors are integrated on a common substrate. As a result of this manufacturing step, transistors provided at one side of the substrate are formed as a lateral structure, thereby resulting in decreasing of current amplification factor in each transistor.
Generally, the current amplification factor in a PNP transistor becomes lower than that in the NPN transistor which is formed on the substrate as a planar structure. In order to improve the current amplification factor, it has been proposed to form the PNP and NPN transistors as a composite transistor on the substrate. This composite transistor can be obtained by connecting the transistors in Darlington arrangement. In this case, however, there is a possibility that the muting range may be set unsymmetrically with respect to the intermediate frequency of 10.7 MHz due to the difference in current amplification factor. Furthermore, since the outputs of the respective NPN and PNP transistors have opposite polarities to each other, the conventional muting range setting circuit is disadvantageous in that it is necessary to deliver the outputs of the respective transistors to the gate driving circuit independently. As a result, the circuit becomes overly complicated in construction.